1. Field of the Invention
The present invention relates to a system and method for joint restoration by extracapsular means, and more particularly to a system and method for corrective displacement of bone by extracapsular means.
2. Background Art
In the typical patient with osteoarthritis (OA) of the weight bearing joints, the disease is often described in terms of the missing articular material that covers the joint surface. In more advanced stages of the disease, the subchondral bone which underlies the articular material is often described as being “eroded” or deformed. In the typical patient with moderate to severe stages of the disease, the combination of missing articular material and deformed subchondral bone can lead to the joint having excessive joint laxity where there is excessive joint spacing, often 2-6 millimeters (mm), and occasionally as much as 10 mm. The areas of lost articular material and deformed subchondral bone in a typical patient with medial compartment OA of the knee are shown in FIGS. 1A and 1B. As the articular covering wears away, the loading in the joint changes towards excessive in the joint compartment that is losing the articular surface. As the load is increasing, the subchondral bone reacts to the increasing loads by changing its shape, thus adapting to handle the increasing loads. It will often thicken in the area directly under the areas of highest loads, and will generate osteophyte formation to increase the area of bone carrying the increased loads.
It has been demonstrated in literature that bone is constantly resorbing and rebuilding in response to biophysical stimuli—see, e.g., Chuanyong Qu, et al., A Hypothetical Mechanism of Bone Remodeling and Modeling Under Electromagnetic Loads, Biomaterials 27, 4050-4057 (2006). Osteocytes sense the increased strain environment, and respond accordingly. When bone tissue is damaged as in the micro-cracking that occurs in the presence of excessive stress or strain, osteoclasts remove the necrotic osteocytes. This activates growth factors held in the osteocytes, such as bone morphogenic protein (BMP) or transforming growth factor (TGF) beta 1. These growth factors are then released into the bone fluid, subsequently stimulating osteoblasts, which in turn, start the process of manufacturing new bone. Id. A strain response threshold limit is that point where the bone will react to the loads and begin to remodel. This is somewhat analogous to an industrial manufacturing technique known as “Incremental Sheet Forming” (ISF). This technique is used to form complex shapes from flat metal without the use of tooling. A “Forming Limit Diagram” (FLD) is created that represents the local limit strains. Strains above the limit represent failure, and below the limit they represent deformation of the material. Deformation limits utilizing this technique are much higher than using the macroscopic method represented by matched tool presses—see, e.g., L. Lamminen, et al., Incremental Sheet Forming with an Industrial Robot, Materials Forum 29, 331-335 (2005).
Similarly, when bone is measured on a large scale, it exhibits very classical (single elastic constant) behavior, but when the scale is reduced down to the trabecular level or below, the behavior becomes much more viscoelastic in nature, and tends to follow a Cosserat (multiple elastic constants) curve. This allows for much higher than predicted (by the classical approach) strain limits before failure occurs—see, e.g., Rod Lakes, On the Torsional Properties of Single Osteons, adapted from J. Biomechanics 28, 1409-1410 (1995). In order for bone formation to be initiated, the magnitude of mechanical strain of the bone must surpass some threshold. Therefore, for restorative remodeling to occur, this threshold must be exceeded, while not causing failure—see, e.g., Yeou-Fang Hsieh, et al., Mechanical Loading of Diaphyseal Bone In Vivo: The Strain Threshold for an Osteogenic Response Varies with Location, J. of Bone and Mineral Research 16, 2291-97 (2001).
In general, bones are made up of a number of different types of osseous material—e.g., trabecular (cancellous), subchondral, and cortical bone. An example of cortical bone is found in the shaft of a femur. Trabecular bone can be found inside the condylar region of a femur, and alongside the cortical bone. The trabecular bone transfers the loads from the subchondral bone to the cortical bone, and the subchondral bone is that bone which supports the articular regions of the joint surfaces. Each different type of bone may undergo different deformation mechanisms. For example, cortical bone in particular exhibits “cement line slippage” between the osteons, which accounts for an ISF type (almost viscoelastic) behavior when applied to localized regions. This is typically considered the reason bone is a “tough, non-brittle” material. It is also a response that is dependent on the direction of the applied load—a result of the oriented structure of bone—see, e.g., Rod Lakes, On the Torsional Properties of Single Osteons, adapted from J. Biomechanics 28, 1409-1410 (1995).
One of the mechanisms of bone deformation is “creep”. Creep is a viscoelastic response defined as a time dependent strain under constant load. At sufficiently high stress levels, deformation will occur with time, leading to “creep-failure”, or deformation that does not recover once the load is removed. The creep response of bone is significantly larger in younger bones as compared to older bones. In the mature skeleton, osteogenesis is initiated only if a mechanical load is applied; however, the bone quickly gets desensitized to mechanical loading and stops responding. Therefore, static loading may not be as effective in remodeling the bones older patients, as compared to younger patients—see, e.g., P. Zioupos and J. D. Currey, Changes in the Stiffness, Strength, and Toughness of Human Cortical Bone with Age, Bone 22(1), 57-66 (1998).
In addition to the magnitude of the stress, the rate of loading can also significantly affect the strain experienced by the bone. For example, a more rapid load onset results in a more rapid bone change. Conversely, a slower application of a load results in a smaller change, but thickening of the bone to handle the higher stress. Thus, a static load may build more dense bone, but a dynamic load may cause greater overall deformation of the bone. Another parameter that can affect the strain response of the bone is the number of cycles during which a load is applied—although this does not seem to have as pronounced an effect as changing the magnitude of the load—see, e.g., C. Rubin, et al., Mechanical Strain, Induced Noninvasively in the High-Frequency Domain, Is Anabolic to Cancellous Bone, But Not Cortical Bone, Bone 30, 445-452 (2002) and D. B. Burr, et al., Bone Remodeling in Response to In Vivo Fatigue Microdamage, J. Biomechanics 18(3), 189-200 (1985). In addition, variation in rest periods—i.e., the length of time between either cyclically or statically applied loads—can also affect bone response. For example, rest periods may be required in order for the bone to respond to loads. Such rest periods can significantly increase the bone's anabolic response to mechanical loading—see, e.g., Charles H. Turner and Alexander G. Robling, Exercise as an Anabolic Stimulus for Bone, Current Pharmaceutical Design 10(21), 2629-41 (2004) and Sekou Singare, et al., The Effect of Latency on Bone Lengthening Force and Bone Mineralization: An Investigation Using Strain Gauge Mounted on Internal Distractor Device, Biomedical Engineering Online 5:18 (2006).
Conventional treatments of bone loss resulting from OA all have inherent limitations. For example, one such treatment used as a response to this loss of bone and the overlying cartilage is to remove even more subchondral bone, and replace it with a metal and plastic implant—the metal acting as a substitute for the bone, and the plastic acting as a substitute for the cartilaginous bearing surface. The unicompartmental knee replacement (UKR) and total knee replacement (TKR) are typical examples of such bone cutting treatment modalities. A high tibial osteotomy or femoral osteotomy may correct the angular misalignment by altering the load path with resection of bone elements extraneous to the joint capsule, but it does not address the issue of the excessive joint space. An interpositional spacer (for example, as described in U.S. Pat. Nos. 6,206,927 and 6,558,421) addresses the excessive joint space with the thickness of the device without requiring bone resection, but may still require the removal of some remaining articular material in order to achieve an adequate bearing surface for the device.
Therefore, a need exists for a system and method of corrective bone displacement that overcomes the limitations of prior art systems and methods by taking advantage of the bone's ability to remodel in the presence of an applied load.